It is well known to those skilled in the art that gelled or crosslinked water-soluble polymers are useful in enhanced oil recovery and other oil field operations. They have been used to alter the permeability of underground formations in order to enhance the effectiveness of water flooding operations. Generally, polymers along with an appropriate crosslinking system are injected in an aqueous solution into the formation. The polymers then permeate into and gel in the regions having the highest water permeability.
Because of environmental concerns as well as cost for disposing of a produced brine which is defined as the brine co-produced with oil and gas, it is desirable to utilize the produced brine as the aqueous solution used for the polymers and appropriate crosslinking systems. Use of produced brines eliminates not only the cost associated with acquiring and pre-treating fresh water for use as the aqueous solution but also the disposal cost for the produced brine. Most produced brines are known to be hard brines, i.e., those having a divalent cation concentration greater than 1000 ppm.
Although a chromium(III) salt which is not as toxic as a chromium(VI) salt can be used as crosslinking agent, it is not an environmentally desirable compound and its use may require additional costs to assure the integrity of the injection wells to avoid contamination of ground water sources. Other multivalent metallic compounds such as compounds of zirconium, titanium, ferric or ferrous, aluminum, or combinations of any two or more thereof have been used to produce gels with synthetic water-soluble polymers or natural polymers for various oil-field operations such as, for example, water shut-off and permeability corrections for reservoirs. Usually these metallic ions crosslink gellable polymers through the interaction with the oxygen atoms of the polymer molecules. Therefore, the gellable polymers generally contain some carboxylate groups. Generally, the gellable polymers used such as, for example, polyacrylamide are of high molecular weight and contain high degrees of hydrolysis, i.e., contain 10-30mole % carboxylate groups. However, these high molecular weight and/or high mole % carboxylate groups containing polymers gel almost instantly in the presence of the above-described multivalent metallic compounds. Such fast gelation rate renders the application of gelling compositions containing these polymers and multivalent metallic compounds not useful in many oil-field applications such as, for example, water shut-offs and permeability reductions.
Many processes have been developed to delay the gelation of gelling compositions by adding a gelation delaying agent to the gelling compositions. However, a gelation delaying agent is not inexpensive and a gelation delaying agent often adds appreciable costs to oil field operation.
Therefore, a more environmentally suitable process using a gelling composition that can form stable gels in produced brines for near-wellbore as well as in-depth treatments and that does not require a gelation delaying agent is highly desirable.